DIALLAGE ROCK. 647 



elude small masses of serpentine, as well as to 

 alternate with large bodies of the same rock. It 

 must also be remarked that, in one or two in- 

 stances, it occurs in very thin beds among* the 

 rocks now described, and very widely separated 

 from any other masses of the same substance. 



The internal structure, or rather the texture, 

 of diallage rock, is sometimes merely granular 

 crystalline ; and it therefore breaks, like granite, 

 indifferently, in any direction ; although, from 

 its toughness, with great difficulty. But it is 

 often fissile, or breaks with more ease in one 

 direction than another. The texture then re- 

 sembles that of gneiss ; this effect being the 

 result of a predominant parallelism in the crystals 

 of the diallage. It is very frequently also inter- 

 sected by extremely thin veins, or laminae, of 

 talc, chlorite, or mica ; these being only disco- 

 vered by the yielding of the rock in those parts: 

 and hence it is with great difficulty that a true 

 fracture is procured. Lastly, it is often traversed 

 by veins, resembling those which occur in gra- 

 nite and in hypersthene rock, in which the con- 

 stituent minerals are crystallized in larger forms, 



